Apparatus and method for performing open loop tracking of a signal

ABSTRACT

Apparatus for performing open loop tracking of a signal in a radio occultation instrumentation system including a first satellite arranged with a transmitting means arranged for transmitting a signal with well defined carrier frequency and a second satellite arranged with signal receiving means receiving said signal, wherein said transmitted signal has a ray path from said first satellite to said second satellite which descends into and ascends from the atmosphere, said apparatus including means for providing a control signal to a frequency control unit for signal acquisition included in said receiving means, means for receiving information I(x,v) regarding the position and velocity of the first and second satellite; arrangement for providing a prediction of Doppler shift displacement of a transmitted signal in such a radio occultation system; and method for determining atmospheric conditions from information of Doppler shift displacement in a radio occultation instrumentation system.

TECHNICAL FIELD

[0001] The invention relates to an apparatus for performing open loop tracking of a signal in a radio occultation instrumentation system according to the preamble of claim 1, an arrangement for providing a prediction of Doppler shift displacement of a transmitted signal according to the preamble of claim 5 and a method for determining atmospheric conditions from information of Doppler shift displacement in a radio occultation instrumentation system including a first satellite arranged with a transmitting means arranged for transmitting a signal with well defined carrier frequency and a second satellite arranged with signal receiving means receiving said signal, wherein said transmitted signal has a ray path from said first satellite to said second satellite which descends into and ascends from the atmosphere.

TECHNICAL BACKGROUND

[0002] Radio Occultation instruments using GNSS signals are space borne receivers, which provides information, regarding Doppler shift of signals transmitted from for instance a GPS satellite having well defined position in a well-defined orbit and a transmitter with a well-defined frequency. The signal from the GPS satellite is received, after having crossed the atmosphere, by a receiver at a Low Earth Orbiting satellite. The Doppler shift of the received signal is measured and from this measurement, vertical profiles of the temperature, pressure and density in the atmosphere can be derived. The basic function of Radio Occultation instrument is thus to receive signals that have crossed the atmosphere at varying altitudes, acquisition of such signals after having crossed dense tropospheric layers often with large refractivity causing dynamics in amplitude and phase. GNSS radio Occultation as an atmospheric sensing tool with advantages regarding all weather capability because clouds do not block the signals. The method provides for high vertical resolution from 1,5 km in the stratosphere to 0,2 km in the troposphere. The accuracy of the retrieved temperature is in the range of 1K. Furthermore the method provides for long-term consistency, which is essential for climate change monitoring.

[0003] A description of how the measurement principle for Radio Occultation for a setting satellite is performed is done with reference to FIG. 1. The measurement is performed according to the following:

[0004] An instrument acquires and tracks the GPS signal at two frequencies L1 and L2. The tracking is performed when a ray path 1 from the satellite 2 carrying the transmitter to the low earth orbiting satellite 3 does not pass the upper parts of the atmosphere. This position is indicated with P1 in FIG. 2. When the ray path 1 descends into the atmosphere, the carrier phase is measured and the Doppler shift is calculated from the measured carrier. The phase measurements are transmitted to a receiving station 4 positioned on the earth. The Doppler shift recorded together with information regarding the position and the velocity of the satellites is used for determining the directions of the reception and transmission. The position of the satellite 2 carrying the transmitter is denoted by X_(G) and the position of the low earth orbiting satellite is denoted by X_(L), where X_(G) and X_(L) are vectors. A further vector x is defined as the positions of both satellites, that is X=(X_(G),X_(L)). In the case of a spherically symmetric atmosphere the central part of the ray path, that is a portion 5 of the ray path around the point where the distance to the earth is at a minimum, is symmetric. Therefore the distances, known as impact parameters a, from the earth centre to ray asymptotes 6,7 at the respective impact point 8,9 are equal. The impact points 8,9 are situated on the ray path 1 where a plane perpendicularly arranged to a ray asymptote 6,7 passes through the centre of the earth. Furthermore the ray asymptotes cross each other at an angle α, which is defined as refraction angle. The impact parameters a and the refraction angle α can be calculated from the Doppler shift using only measurement geometry. Thus, the refraction angle α can be derived as a function of the impact parameter: When performing measurements, the dispersive behaviour of the ionosphere and the non-dispersive behaviour of the neutral atmosphere is used to enable determination of the refraction angle contribution from each of these layers. Further use of the symmetric atmosphere condition allows for determination of the refraction index profile α(r) as a function of the earth radius from the refraction angle α by using the Abel transform. The refraction index profile α(r) depends on the air pressure, temperature and water vapour content, which parameters are retrieved using the gas equation and the hydrostatic equilibrium equation.

[0005] A more detailed presentation of how radio occultation is performed is provided in P Hoeg et al., “The Derivation of Atmospheric Properties by Radio Occultation”, Danish Meteorological Report 95-4, 1994, E. R. Kursinsky et al., “Observing Earth's Atmosphere with radio Occultation Measurements using the GPS”, Journal of Geophysical research, 102, no. D19 October 1997, W. G. Melbourne et al., “The Application of Spaceborne GPS to Atmospheric Limb Sounding and Global Change Monitoring”, JPL Publication 94-18, 1994 and C. Rocken et al., “Analysis and validation of GPS/MET Data in the Neutral Atmosphere”, Journal of Geophysical Research, 1998, which all are incorporated by reference.

[0006] As the ray path transverse the atmosphere at lower altitudes, the ray is gradually more bent, attenuated and spread, due to the increase in refractivity. At high altitudes, where the signal is strong, the receiver locks on to the signal carrier, while at the lower altitudes, where the signal is weak and has large dynamic properties tracking can sometimes not be performed. Therefore, there is a need for an aided tracking that is model based. Attempts have been made to perform model based tracking, however, the suggested methods have shown to include difficulties because of the large impact from atmospheric Doppler shift at the lowest altitudes, which have resulted in loss of signal or inaccuracies in the measurement results for low altitudes, which is altitudes less than 5 km above the surface of the earth.

DISCLOSURE OF THE INVENTION

[0007] An object of the invention is to provide apparatus for performing open loop tracking of a signal in a radio occultation instrumentation system, where the effects of the large variation of the atmospheric Doppler shift at low altitudes are mitigated. This object is achieved by an apparatus for performing open loop tracking of a signal in a radio occultation instrumentation system according to the characterising portion of claim 1. A further object of the invention is to provide an arrangement for providing a prediction of Doppler shift displacement of a transmitted signal in a radio occultation system, where the effects of the large variation of the atmospheric Doppler shift at low altitudes are mitigated. This object is achieved by an arrangement for providing a prediction of Doppler shift displacement of a transmitted signal in a radio occultation system according to the characterising portion of claim 5. A still further object of the invention is to provide a method for determining atmospheric conditions from information of Doppler shift displacement in a radio occultation instrumentation system, where the effects of the large variation of the atmospheric Doppler shift at low altitudes are mitigated. This object is achieved by method for determining atmospheric conditions from information of Doppler shift displacement in a radio occultation instrumentation system, according to claim 9.

[0008] The objects set out above are all achieved by the fact that the means for providing a control signal is arranged for providing parametric value Q(x,v,α(x)) which asymptotically converge towards a function F(x,v) dependent on the satellite positions and velocities and independent of atmosphere conditions when the ray path leaves the atmosphere and asymptotically towards a fixed value when the ray path approaches the surface of the earth, wherein said parametric value is arranged for providing signal acquisition by said frequency control unit. Using the parametric value with these properties it is possible to arrange the open loop tracking to adequately follow the phase shift in the low regions and thereby obtain good measurement results. This is due to the fact that even if the atmospheric Doppler is very large at low impact altitudes, the impact altitude asymptotically converges to a constant value at low impact altitudes. The signal acquisition is performed by the signal receiving means where an antenna receives a signal and a down converting circuit down converts the signal received by the antenna with a model controlled frequency generated by a local oscillator included in said frequency control unit using said parametric value. Furthermore, the signal receiving means includes a sampling circuit arranged for sampling the down converted signal with a sampling frequency wide enough to accommodate the bandwidth of the received signal together with the deviation of the centre frequency of the received signal and the centre frequency of a signal predicted by said parametric value.

[0009] The straight-line tangential altitude is defined as the distance between the ellipsoidal tangent point and the straight-line tangent point. The ellipsoidal tangential point is defined as a point on the earth surface where the straight line of sight between the low orbiting satellite and the satellite carrying the transmitter is parallel to the tangent of the surface of the earth, and lies in the plane spanned by two satellites and the centre of the earth. The straight-line tangent point is defined as the point of a straight line between the low earth orbiting satellite and the satellite carrying the transmitter that is closest to the ellipsoidal tangent point.

[0010] In particularly preferred embodiments the straight-line altitude is used as a basis for the calculation of the parametric value. This is particularly advantageous since the straight line altitude continues to decrease to large negative numbers when the impact altitude asymptotically converges to a constant value at low impact altitudes, thereby the effects of large variation in atmospheric Doppler can be compensated with the span of the variable.

SHORT DESCRIPTION OF THE FIGURES

[0011] An embodiment of the invention will be described in further detail together with references to appended figures, where,

[0012]FIG. 1 shows an outline of a radio occultation instrumentation system,

[0013]FIG. 2 shows definitions of the ray geometry,

[0014]FIG. 3 shows an embodiment of an arrangement for performing a prediction of Doppler shift displacement,

[0015]FIG. 4 is RO geometry showing refraction angle and impact parameter, and

[0016]FIG. 5 is a Model for NRIPA as a function of SLTA.

DETAILED DESCRIPTION OF THE INVENTION

[0017] In FIG. 1 a radio occultation instrumentation system is shown. The system includes a first satellite 2 arranged with a transmitting means arranged for transmitting a signal with well defined carrier frequency and a second satellite 3 arranged with signal receiving means receiving said signal, wherein said transmitted signal has a ray path 1 from said first satellite 3 to said second satellite 3 which descends into and ascends from the atmosphere 12. The first satellite 2 is preferably a GPS satellite having a well-defined position in a well-defined orbit around the earth. For detailed information about GPS systems is referred to E. D Kaplan., “Understanding GPS—Principles and Applications”, Artech House 1996. Furthermore the frequencies of the transmitting means are well defined. In a practical existing system, these frequencies are 1,227 and 1,575 GHz, but other frequencies could be used. The second satellite 3 is preferably a low earth orbiting satellite. This second satellite 3 receives the signal transmitted from the first satellite 2 and measures the Doppler shift displacement of the signal. Information regarding the Doppler shift displacement is then transmitted to a receiving station 4 positioned on ground. The signal is further processed on the ground in order to determine atmospheric conditions such a temperature, density and pressure as a function of the altitude.

[0018] Radio occultation systems and its general apparatus included in such systems are known to the skilled man in the art, for example through P Hoeg et al., “The Derivation of Atmospheric Properties by Radio Occultation”, Danish Meteorological Report 95-4, 1994, E. R. Kursinsky et al., “Observing Earth 's Atmosphere with radio Occultation Measurements using the GPS”, Journal of Geophysical research, 102, no. D19 October 1997, W. G. Melbourne et al., “The Application of spaceborne GPS to Atmospheric Limb Sounding and Global Change Monitoring”, JPL Publication 94-18, 1994 and C. Rocken et al., “Analysis and validation of GPS/MET Data in the Neutral Atmosphere”, Journal of Geophysical Research, 1998, and will therefore not be described further in detail.

[0019] At high altitudes, that is when a ray path which is described in further detail in FIG. 2 does not pass the atmosphere on its way from the first satellite 2 to the second satellite 3, where the signal is strong, the receiver locks on to the carrier signal and phase locked loop tracking of the carrier signal is performed. The second satellite is provided with a system for Phase lock loops for carrier acquisition. Such systems are known to the skilled man in the art and will therefore not be described in more detail. At the lower altitudes, where the signal is weak and has large dynamic properties tracking can not be performed. Therefore, there is a need for a model-based tracking.

[0020] The second satellite 3 is provided with an apparatus for performing open loop tracking of a signal. The apparatus for performing open loop tracking includes means for providing a control signal to a frequency control unit for signal acquisition included in receiving means, arranged on said second satellite 3. The frequency control unit for signal acquisition is of a type known to the skilled in the art and will therefore not be described in further detail. The frequency control unit preferably is arranged as a part of the system for Phase locked loop signal acquisition arranged on the second satellite. The apparatus for performing open loop tracking includes means for receiving information I (x,v) regarding the position and velocity of the first and second satellite. Here x is a 6 dimensional vector containing the positions of both the first satellite 2 and the second satellite 3 and v is a 6 dimensional vector containing the velocities of both the first satellite 2 and the second satellite 3. A coordinate system with origo at the mid point of the Earth is preferably used, but any other coordinate system might be used. The open loop tracking is performed using the information I (x,v) regarding the position and velocity of the first and second satellite in a manner, which is described with reference to FIG. 3.

[0021] In FIG. 2 the geometry of a ray path 1 from the first satellite 2 to the second satellite. The ray path follows a first ray asymptote 6 until the ray is bent in the atmosphere, where after it follows a second ray asymptote 7. The first and second ray asymptotes 6,7 crosses each other at a refraction angle α. Impact points 8,9 are situated on the ray asymptotes where a plane perpendicularly arranged to a ray asymptote 6,7 passes through the centre of the earth. A straight line 10 connects the first and the second satellite. The straight-line tangential altitude, SLTA, is defined as the distance between the ellipsoidal tangent point and the straight-line tangent point. The ellipsoidal tangential point is defined as a point on the earth surface where the straight line of sight between the low orbiting satellite and the satellite carrying the transmitter is parallel to the tangent of the surface of the earth. The straight-line tangent point is defined as the point of a straight line between the low earth orbiting satellite and the satellite carrying the transmitter that is closest to the ellipsoidal tangent point.

[0022] In FIG. 3 a block scheme of an apparatus for performing open loop tracking of a signal in a radio occultation instrumentation. The apparatus is generally denoted by 13. The apparatus includes means for receiving information I (x,v) regarding the position and velocity of the first and second satellite. The information regarding the position x of the first and second satellite is used as an input to a first functional block 14 of the apparatus 13 for performing closed loop tracking. A second functional block 15 includes a model of the geometry of the earth. For ray path modelling purposes, the model of the earth is defined as the Earth Reference Ellipsoid (ERE) defined in [WGS84] frame as:

R _(earth) =R _(equator)*(1−f*sin (lat0*π/180)²)

[0023] where

[0024] R_(equator)=6378.137 km,

[0025] f=0.0033528 and

[0026] lat0 is the latitude of the tangent point.

[0027] R_(earth) and we have local spherical symmetry at lat0. Other models known to the skilled in the art may of course be used.

[0028] In the first functional block, the straight line tangential altitude, SLTA, is calculated from the positions x of the first and the second satellite and information regarding the local radius of the Earth provided by the Earth model in the second functional block. The straight line tangential altitude, SLTA, is used as input to a third functional block 16 where a calculation of a nominal ray impact altitude, NRIPA, is performed. The nominal ray impact altitude is defined as the distance from the surface of the earth to the impact points 8,9. The correspondence between the nominal ray impact altitude, NRIPA, and straight-line tangential altitude, SLTA is defined in a map created from calculations of ray path geometry in a normal atmosphere.

[0029] A definition of the correspondence between SLTA and NRIPA is defined as follows:

[0030] The critical part of the Doppler model is the computation of NRIPA as a function of SLTA. These two parameters are defined as:

[0031] NRIPA: Nominal Ray Impact Point Altitude (above Earth Reference Ellipsoid) SLTA: Straight Line Tangent Altitude (distance between SLTP and ETP)

[0032] We define the following expression:

NRIPA(SLTA)=MCC·LOG₁₀(10^(MIPA/MCC)+10^(SLTA/MCC))  (1)

[0033] where

[0034] MCC=35 km (Model Curvature Constant)

[0035] MIPA=3.2 km (Minimum Impact Point Altitude)

[0036] and SLTA and NRIPA are in kilometres. A plot of the model function is shown in FIG. 5.

[0037] In a fourth functional block 17 the nominal ray impact altitude, NRIPA, and the local earth radius, Re are added whereby the distance from the centre of the earth to the impact points 8,9 is calculated. The distance to the impact point a, the positions x of the first and the second satellite and the velocity v of the first and second satellite are used as input parameters to a fifth functional block 18. In the fifth functional block 18 the projection of the Doppler shift displacement in the direction of the ray asymptotes are calculated from the input parameters. The calculated Doppler shift displacement is then transformed to a signal provided to the means for providing a control signal to a frequency control unit for signal acquisition included in the receiving means arranged on the second satellite 3. By using this method of calculation of the Doppler shift displacement a control signal, which is intended to be transmitted to the frequency control unit, is established which provides a parametric value Q(x,v,α(x)) which asymptotically converge towards a function F(x,v) dependent on the satellite positions and velocities and independent of atmosphere conditions when the ray path leaves the atmosphere and asymptotically towards a fixed value when the ray path approaches the surface of the earth. The parametric value is arranged for providing signal acquisition by said frequency control unit.

[0038] In the calculation the following relation between satellite geometry, where circular orbits are assumed for simplicity, impact parameter and doppler shift is used:

[0039] The phase carrier phase measurement provides the Doppler shift Δf_(D): $\frac{\Delta \quad f_{D}}{f} = {{\frac{V_{L}}{c}\cos \quad \phi_{L}} + {\frac{V_{G}}{c}\cos \quad \phi_{G}}}$

[0040] f measurement frequency

[0041] c velocity of light

[0042] The Doppler is related to the impact parameter a (assuming spherical symmetry): ${\lambda \quad \Delta \quad f_{D}} = {{V_{L}\frac{a}{R_{L}}} + {V_{G}\frac{a}{R_{G}}}}$ and  the  bending  angle: $\alpha = {\theta - {\cos^{- 1}\left( \frac{a}{R_{L}} \right)} - {\cos^{- 1}\left( \frac{a}{R_{G}} \right)}}$

[0043] θ is the angular separation between the LEO and the GPS satellite.

[0044] The invention described above is used in an arrangement for providing a prediction of Doppler shift displacement of a transmitted signal in a radio occultation system, which includes the elements of the apparatus for performing open loop tracking as defined in claim 1, except for the means for providing a control signal to a frequency control unit for signal acquisition included in the receiving means arranged on the second satellite 3. Furthermore, the invention is used in a method where open loop tracking is performed following the scheme set out above. That is a parametric value Q(x,v,α(x)) which asymptotically converge towards a function F(x,v) dependent on the satellite positions and velocities and independent of atmosphere conditions when the ray path leaves the atmosphere and asymptotically towards a fixed value when the ray path approaches the surface of the earth is calculated from information I(x,v) regarding the positions of a first and second satellite 2,3 together with information regarding the local radius of the earth. In a preferred embodiment the parametric value obtains this property by using Nominal ray Impact Altitude as a function of Straight Line Tangential Altitude. The parametric value is transformed to an output signal which is transmitted to a frequency control unit for performing signal acquisition. The open loop tracking is followed by the method steps of

[0045] collecting phase information from the tracked transmitted signal;

[0046] transmitting information regarding the collected transmitted information;

[0047] track and extract carrier phase from the collected transmitted information;

[0048] receiving information at ground regarding the collected transmitted information;

[0049] performing calculations determining atmospheric conditions from said received signal. 

1. An apparatus for performing open loop tracking of a signal in a radio occultation instrumentation system including a first satellite arranged with a transmitting means arranged for transmitting a signal with well defined carrier frequency and a second satellite arranged with signal receiving means receiving said signal, wherein said transmitted signal has a ray path from said first satellite to said second satellite which descends into and ascends from the atmosphere, said apparatus including means for providing a control signal to a frequency control unit for signal acquisition included in said receiving means, means for receiving information (I(x,v)) regarding the position and velocity of the first and second satellite, wherein said means for providing a control signal is arranged for providing parametric value (Q(x,v,α(x))) which asymptotically converge towards a first function (F(x,v)) dependent on the satellite positions and velocities and independent of atmosphere conditions when the ray path leaves the atmosphere and wherein said parametric value (Q(x,v,α(x))) asymptotically converge towards a second function (G(x,v,r)) dependent of the local earth radius and independent of atmosphere conditions when the ray path approaches the surface of the earth, wherein said parametric value is arranged for providing signal acquisition by said frequency control unit.
 2. Apparatus for performing open loop tracking according to claim 1, wherein said parametric value (Q(x,v,α(r))) corresponds to the Doppler shift displacement of the transmitted signal.
 3. Apparatus for performing open loop tracking according to claim 1, wherein, in order to determine the parametric value (Q(x,v,α(r))), the apparatus is arranged for determining the position of local impact points (P(x)) of the ray path from said information (I(x,v)).
 4. Apparatus for performing open loop tracking according to claim 3, wherein the apparatus is arranged for determine the parametric value (Q(x,v,α(r))) from straight line tangent altitude based on the position of the local impact points (P(x)) and information regarding the position of the first and second satellites (I(x,-)).
 5. An apparatus for performing open loop tracking according to claim 1, wherein the signal receiving means further includes an antenna arranged for receiving a signal and a down converting circuit arranged for down converting the signal received by the antenna with a model controlled frequency generated by a local oscillator included in said frequency control unit using said parametric value.
 6. Apparatus for performing open loop tracking according to claim 5, wherein the signal receiving means further includes a sampling circuit arranged for sampling the down converted signal with a sampling frequency wide enough to accommodate the bandwidth of the received signal together with the deviation of the centre frequency of the received signal and the centre frequency of a signal predicted by said parametric value.
 7. An arrangement for providing a prediction of Doppler shift displacement of a transmitted signal in a radio occultation system including a first satellite arranged with a transmitting means arranged for transmitting a signal with well defined carrier frequency and a second satellite arranged with signal receiving means receiving said signal, wherein said transmitted signal has a ray path from said first satellite to said second satellite, which descends into and ascends from the atmosphere, said arrangement including means for receiving information (I(x,v)), regarding the position and velocity of the first and second satellite and means for providing a control signal, to a frequency control unit for signal acquisition, based on said information (I(x,v)) wherein said means for providing a control signal is arranged for providing parametric value (Q(x,v,α(x))), which asymptotically converge towards a function (F(x,v)) dependent on the satellite positions and velocities and independent of atmosphere conditions when the ray path leaves the atmosphere wherein said parametric value (Q(x,v,α(x))) asymptotically converge towards a function (G(x,v,r)) dependent of the local earth radius and independent of atmosphere conditions when the ray path approaches the surface of the earth.
 8. Arrangement according to claim 7, wherein said parametric value (Q(x,v,α(r))) corresponds to the Doppler shift displacement of the transmitted signal.
 9. Arrangement according to claim 7, wherein, in order to determine the parametric value (Q(x,v,α(r))), the arrangement is arranged for determining the position of local impact points (P(x)) of the ray path from said information (I(x,v)).
 10. Arrangement according to claim 7, wherein the arrangement is arranged for determine the parametric value (Q(x,v,α(r))) from straight line altitude based on the position of the local impact points (P(x)) and information regarding the position of the first and second satellites (I(x,-)).
 11. Arrangement according to any of claim 7, wherein the signal receiving means further includes an antenna arranged for receiving a signal and a down converting circuit arranged for down converting the signal received by the antenna with a model controlled frequency generated by a local oscillator included in said frequency control unit using said parametric value.
 12. Apparatus for performing open loop tracking according to claim 11, wherein the signal receiving means further includes a sampling circuit arranged for sampling the down converted signal with a sampling frequency wide enough to accommodate the bandwidth of the received signal together with the deviation of the centre frequency of the received signal and the centre frequency of a signal predicted by said parametric value.
 13. Method for determining atmospheric conditions from information of Doppler shift displacement in a radio occultation instrumentation system including a first satellite arranged with a transmitting means arranged for transmitting a signal with well defined carrier frequency, a second satellite arranged with signal receiving means receiving said signal and a ground based station arranged with a receiver for receiving information regarding the Doppler shift displacement, wherein said transmitted signal has a ray path from said first satellite to said second satellite which descends into and ascends from the atmosphere, said method including the following method steps: performing open loop tracking of a transmitted signal using an apparatus according to any of claims 1-6; collecting phase information from the tracked transmitted signal; transmitting information regarding the collected transmitted information; track and extract carrier phase from the collected transmitted information; receiving information at ground regarding the collected transmitted information; performing calculations determining atmospheric conditions from said received signal. 